1. Field of the Invention
The present invention relates to a level shifter which is to be used in conjunction with a circuit that requires a reference voltage which is also used as a reference for the level shifter.
2. Discussion of the Related Art
FIG. 1 shows a conventional structure of a level shifter used in conjunction with, for example, a resistor-tree digital-to-analog converter (DAC) 10. A resistor-tree DAC needs a precision reference voltage Vref of a predetermined value, for example 2 volts. This reference voltage is provided at low impedance, for example, as shown, by a band-gap reference source 12 followed by an operational amplifier 14 whose gain is set by two resistors 16, 17 for obtaining the desired value of the reference voltage Vref.
A DAC is usually supplied with a relatively low voltage, for example 5 volts, and its output voltage Vi is usually amplified and level shifted to adapt it to a circuit supplied at a higher voltage, for example 12 volts.
A level shifter needs an accurate reference voltage for determining the amount by which its input voltage (Vi) should be shifted with respect to a generally inaccurate common-mode voltage Vcm.
The level shifter of FIG. 1 is essentially a differential amplifier 20 including an operational amplifier 22. The inverting input of operational amplifier 22 is connected to its output Vo through a resistor 24 and receives a reference voltage qVref through a resistor 25. The non-inverting input of operational amplifier 22 receives the common-mode voltage Vcm through a resistor 27 and the voltage Vi to be shifted through a resistor 28. By choosing a normalized value 1 for resistors 25 and 28, and a normalized value k for resistors 24 and 27, the gain of the differential amplifier 20 is set to k.
With this arrangement, the output voltage Vo of the level shifter is expressed by: EQU Vo=Vcm+k(Vi-qVref).
The differential amplifier 20 of FIG. 1 has relatively low impedance inputs, whereby its input voltages, qVref and Vi, should be provided by low impedance sources.
The input voltage Vi of the differential amplifier 20 is provided from the output voltage of the resistor-tree DAC 10 through a unity gain operational amplifier 30.
In most cases, the reference voltage Vref needed by the resistor-tree DAC 10 is not adapted to the level shifter and must be either attenuated or amplified. In FIG. 1, reference voltage Vref is attenuated by a bridge of two resistors 32, 33 connected to ground. The values of resistors 32 and 33 are chosen so that the midpoint of this bridge provides the desired reference voltage qVref for the level shifter, where q is the ratio between the reference voltage for the level shifter and reference voltage Vref. The normalized values of resistors 32 and 33 are respectively 1-q and q. The midpoint of the resistor bridge being at relatively high impedance, its voltage is buffered by a unity gain operational amplifier 35 which provides the final reference voltage for the level shifter.
A drawback of the level shifter of FIG. 1 is that precision in its reference voltage qVref is lost due to the inherent offset voltage of operational amplifier 35.